Various solutions and multiple optical combinations have been tried in the past to achieve coupling between a phototransducer and an optical fiber.
Some of those solutions are analyzed in the document "Progress in monomode optical fiber interconnection devices" published in "Journal of Lightwave Technology", vol. LT-2, No. 3, pp. 217-227, June 1984.
In French patent application filed under the number 91 11493, the Applicant has itself proposed an optical system obtained by means of a method comprising the following steps:
cleaving an optical fiber; PA1 a piece of another fiber is subjected to heating and is stretched axially until it breaks; PA1 aligning the stretched piece of fiber on the cleaved end of the optical fiber and putting the tip of the stretched piece of fiber into mechanical contact with the core at the cleaved end of the optical fiber; and PA1 welding a portion of the tip of the stretched piece of fiber onto the core of the cleaved end of the optical fiber in order to form a lens of desired curvature. PA1 cleaving a polarization-maintaining optical fiber; PA1 stretching a piece of another fiber axially to rupture, while applying heat thereto; PA1 alining the stretched piece of fiber on the cleaved end of the polarization-maintaining optical fiber and mechanically putting the tip of the stretched piece of fiber into contact with the core of the cleaved end of the polarization-maintaining optical fiber; and PA1 welding a portion of the tip of the stretched piece of fiber onto the core of the cleaved end of the polarization-maintaining optical fiber to form a lens of desired curvature.
Nevertheless, none of those solutions gives complete satisfaction when, as is often the case, the phototransducers and the optical fibers possess differing optical modes. The optical modes of phototransducers are generally small and elliptical, typically 3.times.0.5 .mu.m.sup.2, whereas the modes of optical fibers are generally circular and larger, typically having a diameter of 8 .mu.m to 11 .mu.m.
One of the key points of coupling remains matching modes between optical fibers and phototransducers.
In an attempt to improve such coupling, certain elliptical lensing techniques have been proposed.
For example, proposals have been made in the article by H. M. Presby and C. R, Giles entitled "Asymmetric fiber microlenses for efficient coupling to elliptical laser beams", published in IEEE Photonics Technology Letters, Vol. 5, No. 2, February 1993, to use a CO.sub.2 laser to mechanically machine the end of the fiber to be coupled.
Proposals have also been made in the article by W. Hunziker, E. Bolz, and H. Melchior entitled "Elliptically lensed polarization maintaining fibers", published in Electronics Letters, Aug. 13, 1992, Vol. 28, No. 17, to chemically etch a polarization-maintaining fiber, and then polish the core thus put into relief by melting.
The techniques proposed in those two documents do not give complete satisfaction either. The processes of mechanical machining and of chemical etching are quite difficult to perform.